The I/O Unit Usage Report lists all of the external I/O units used in the application that are denoted by either a simple number or a variable name. A subsequent unit selection will display the I/O performed on that unit by routine and statement used, thus documenting your application's I/O sites.
Units denoted by variable name assume the name is merely used as an alias for the unit and remains fixed during the run. This is considered good programming practice, particularly when portable code is required and when mnemonic names are used to describe the target file. Obviously, if the name is reset in your application with a different unit number, the analysis will be less meaningful.
NOTE: Non-standard FORTRAN-77 I/O statements, I/O performed in library routines, units designated with expressions (e.g., U+1), or internal I/O are not tracked in this report.